Add HDD to ispconfig

Hi, i've buyed a server So You Start from OVH with 3 hard disk. I've 3 hard disk of 2TB, so 6TB in total.
I've installed Debian 9 with IspConfig, but i see only 1 hard disk mounted. (I've attached screenshot)
How to add the other 2 hard disk? As we have to upload many files like 3-4 TB, I wish that once the space is finished, it start saving on the other hard drives. It's possible?

but i want to know: how to add them on IspConfig? It's automatic? When a partitions is full it in automatic store file in free partitions? I've only 1 website on ispconfig with Wordpress, i've to add 4TB of Media in Wordpress.

Disks are not added in ISPConfig. Add disks in operating system, Debian in your case.

It's automatic

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No.
If you already have 4 Terabytes data, seems the only way to set up the 3 disks of 2T each is to make them into one 6 T disk. Drawback is there is no redundancy, and if one disk goes bad all data is lost.
Where are backups going? There is no space in that host for them.